Pellet-type foams of non-crosslinked polypropylene resin having lower melting point and process and device for producing the same and molded foams therefrom

ABSTRACT

The present invention relates to a pellet-type non-crosslinked polypropylene foam having a melting point of 125 to 140° C., and a process and device for producing said foam. Since the pellet-type foams of non-crosslinked polypropylene of the present invention has a lower melting point and a closed cell content of 80% or more, it is advantageous to mold such foams. The present invention also relates to an article molded from the above pellet-type non-crosslinked polypropylene foams.

RELATED APPLICATIONS

This application claims priority to Korean Patent Application No.2001-0028241 filed May 23, 2001, Korean Patent Application No.2001-0028242 filed May 23, 2001, Korean Patent Application No.2001-0028243 filed May 23, 2001, Korean Patent Application No.2002-0023836 filed Apr. 30, 2002, Korean Patent Application No.2002-0026539 filed May 14, 2002 and PCT Application No. PCT/KR02/00915filed May 15, 2002 entitled Pellet-Type Foams of Non-CrosslinkedPolypropylene Resin Having Lower Melting Point and Process and Devicefor Producing the Same and Molded Foams Therefrom.

TECHNICAL FIELD

The present invention relates to a pellet-type non-crosslinkedpolypropylene foam having a melting point of 125° C. to 140° C., aprocess for producing said foam, a device for realizing said process andan article molded from said foam.

BACKGROUND ART

Due to its excellent mechanical strength and cushioning properties, apolypropylene resin foam is widely used as a packaging material, abuilding material, a heat shield material and the like. However, sincepolypropylene has a high crystallinity and a low melt viscosity and isdifficult to cross-link, it has hereto been quite difficult to obtain ahighly expanded product from the polypropylene.

U.S. Pat. No. 5,527,573 (issued Jun. 18, 1996) discloses extrudedclosed-cell polypropylene resin foam and several methods for producingsaid foam. The foam of the US patent is in a plank form and has aminimal cross-sectional area of about 5×2.54 square centimeters, aminimal thickness of 12.7 millimeters and a density of about 5 poundsper a cubic foot. The form and properties make it difficult to mold thefoam into desired shaped articles. U.S. Pat. No. 6,051,617 (issued Apr.19, 2000) discloses a foamed polypropylene resin particle useful formolding a foamed, molded article and a method of preparing the same.However, the foamed polypropylene resin particle is prepared by graftinga vinyl comonomer to polypropylene resin particles to form the modifiedcopolymer resin particles and foaming the modified copolymer resinparticles. U.S. Pat. No. 6,077,875 (issued Jun. 20, 2000) disclosesfoamed and expanded beads of a polypropylene resin for molding preparedfrom a non-crosslinked propylene-ethylene random copolymer. The foamedbeads of the US patent has an open cell content of at most 40%, mostpreferably 25% and has a melting point of at least 141° C.

DISCLOSURE OF INVENTION

A non-crosslinked polypropylene resin is advantageous as it can berecycled and the pellet-type foam produced from the resin can be easilymolded. However, the pellet-type foam obtained through extrusion of thenon-crosslinked polypropylene resin contains open cells to no smallextent and thus is useless. For its practicability, the pellet-type foammust contain a great amount of closed cells for mechanical strength. Itis only JSP Corporation of Japan throughout the world that successfullycommercially produces a pellet-type foam from a non-crosslinkedpolypropylene resin. However, while it is recognized that thepellet-type non-crosslinked polypropylene foam having lower meltingpoint are highly valuable owing to its easy molding and excellentrecycling, a pellet-type non-crosslinked polypropylene foam having amelting point of 140° C. and less has not yet been produced.

Therefore, an object of the present invention is to provide apellet-type polypropylene resin foam with a melting point of 140° C. orless which is produced from a non-crosslinked polypropylene resin toensure a recyclability, has a high content of closed cells to provide asatisfactory mechanical strength and can be molded into various shapedpackaging materials, and a method for producing the same. The inventorshave successfully produced pellet-type foams of non-crosslinkedpolypropylene with a melting point of 125 to 140° C., which comprisesabout at least 40% of closed cells by providing a tandem extruder with aplurality of temperature zones having specifically varied temperatures,making the non-crosslinked polypropylene resin melt having a meltingpoint of 138 to 140° C. flow through the temperature zones, mechanicallyhomogenizing the polypropylene resin melt passed through such zones at alower temperature of 120 to 130° C., expanding the homogenized meltthrough a plurality of holes formed in the dies under pressure, andcutting expanded foams discharged from the holes of the dies.

In one aspect, the present invention provides a pellet-typenon-crosslinked polypropylene foam having a melting point of 125 to 140°C.

In another aspect, the present invention provides a method for preparinga pellet-type non-crosslinked polypropylene foam having a melting pointof 125 to 140° C., comprising steps of; (a) extruding a non-crosslinkedpolypropylene random copolymer having a melting point of 138 to 140° C.through a tandem extruder; the tandem extruder consisting of the firstextruder divided into the first temperature zone in which a temperatureof 147 to 153° C. is set, the second temperature zone in which atemperature of 167 to 172° C. is set, the third temperature zone inwhich a temperature of 168 to 172° C. is set, the fourth temperaturezone in which a temperature of 218 to 225° C. is set, the fifthtemperature zone in which a temperature of 197 to 203° C. is set and thesixth temperature zone in which a temperature of 188 to 193° C. is set,the second extruder divided into the first temperature zone in which atemperature of 167 to 173° C. is set, the second temperature zone inwhich a temperature of 147 to 152° C. is set, the third temperature zonein which a temperature of 142 to 147° C. is set, the fourth temperaturezone in which a temperature of 137 to 141° C. is set, the fifthtemperature zone in which a temperature of 137 to 142° C. is set and thesixth temperature zone in which a temperature of 132 to 137° C. is set,and a guide, connecting the first extruder with the second extruder, inwhich a temperature of 248 to 255° C. is set; (b) compulsorily flowingthe extruded material at a temperature of 125 to 140° C. by pumping; (c)homogenizing the extruded material at a temperature of 120 to 130° C.;(d) expanding the homogenized material through a dies; and (e) cuttingthe expanded material to obtain the pellet-type foams.

In another aspect, the present invention provides a device for producinga pellet-type non-crosslinked polypropylene foam having a melting pointof 125 to 140° C.

In another aspect, the present invention provides articles molded frompellet-type non-crosslinked polypropylene foams having a melting pointof 125 to 140° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1 is a view showing the overall structure of the device forproducing pellet-type foams of non-crosslinked polypropylene accordingto the present invention;

FIG. 2 is a plan view showing a cylinder of the first extruder shown inFIG. 1;

FIG. 3 is a sectional view taken along the line A—A in FIG. 2;

FIG. 4 is a schematic view showing the structure of the device forsupplying CO₂;

FIG. 5 is a sectional view showing the inner structure of the pumpingpart and the homogenizing part shown in FIG. 1;

FIG. 6A and FIG. 6B are front views showing each member of thehomogenizing part;

FIG. 7 is a plan view showing the structure of the die part shown inFIG. 1;

FIG. 8 is a sectional view taken along the line B—B in FIG. 7;

FIG. 9 is a sectional view taken along the line A—A in FIG. 8;

FIG. 10A is a side view showing a cylinder of the die part to which acooling device is mounted;

FIG. 10B is a front view of FIG. 10A;

FIG. 11 is a DSC curve of the pellet-type polypropylene foams(Example 1) prepared according to the present invention;

FIG. 12 is a DSC curve of the pellet-type polypropylene foams (Example2) prepared according to the present invention;

FIG. 13 is a DSC curve of RP2400 (polypropylene-polyethylene (3%))copolymer used for preparing the pellet-type polypropylene foams of thepresent invention;

FIG. 14 shows the result of the FT-IR analysis of the pellet-typepolypropylene foams prepared according to the present invention;

FIG. 15 shows the result of the FT-IR analysis of RP2400(polypropylene-polyethylene (3%)) copolymer used for preparing thepellet-type polypropylene foams of the present invention;

FIG. 16A is a photograph taken by an optical microscope at ×100magnification and showing the pellet-type polypropylene foams preparedaccording to the present invention;

FIG. 16B is a photograph taken by an optical microscope at ×400magnification and showing the pellet-type polypropylene foams preparedaccording to the present invention;

Similar reference numbers refer to similar parts throughout severalviews of the drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

Prior to the present invention, it has been impossible to preparepellet-type foams having a melting point of 140° C. or less. Purepolypropylene whose melting point is 138° C. could not be processed at atemperature of 138° C. or less, since it would be cured rapidly at itsmelting point or less. Thus, it has been also considered as beingimpossible to prepare pellet-type foams having a melting point of 140°C. or less.

However, the inventors have developed the pellet-type non-crosslinkedpolypropylene foams which have a melting point of 140° C. or less and anopen cell content of about 20% or less. Such pellet-type foams have beendeveloped by combination and application of a number of discoveries. Forexample, the inventors have found that the content of open cells infoams can be remarkably decreased by using a tandem extruding method asa basis, setting specific temperature conditions for the first andsecond extruding process, and homogenizing the melt resulting from theextruder at a lower temperature of 125 to 130° C. Also, the inventorshave found that only when the temperature during the extrusion andexpansion of the non-crosslinked polypropylene resin having a meltingpoint of 138 to 140° C. is kept within a specific temperature range,closed cells of 80% or more can be formed. Further, the inventor havefound that a melting point of the foams prepared from thenon-crosslinked polypropylene resin having a melting point of 138 to140° C. can be lowered to 138° C. or less by the specific workingconditions according to the present invention.

Now, the method for preparing the pellet type foams of non-crosslinkedpolypropylene according to the present invention will be described indetail. The manufacturing method of the present invention comprisesextruding, pumping, homogenizing, expanding and pelletizing steps.

(1) Extruding

An extruding process according to the present invention can be performedby using a tandem extruder that is commonly used and well known in theart for preparing foams as a basis or its variation. Materials used forpreparing the pellet type foams of non-crosslinked polypropyleneaccording to the present invention comprises a non-crosslinkedpolypropylene random copolymer having a melting point of 138 to 140° C.,a nucleating agent, a foaming agent and an additive, if necessary.

The basic resin of the present invention is the non-crosslinkedpropylene random copolymer having a melting point of 138 to 140° C.Examples of other comonomer copolymerizable with propylene includeethylene, 1-butene, 1-pentent and 1-hexene. The propylene randomcopolymer can be bipolymers such as propylene-ethylene random copolymeror propylene-butene random copolymer or terpolymers such aspropylene-ethylene-butene copolymer. The ratio of other comonomercomponent other than propylene in the copolymer is preferably 0.05 to10% by weight.

The nucleating agent functions to disperse a foaming agent and adjustthe cell size of the foams. Examples of the nucleating agent which canbe used in the present invention include, but are not limited to, sodiumbicarbonate, sodium carbonate, potassium bicarbonate, potassiumcarbonate, ammonium bicarbonate or ammonium carbonate. Sodiumbicarbonate is preferred. The more amount of the used nucleating agentis, the smaller the cell size of the foams is. On contrary, the lessamount of the used nucleating agent is, the larger the cell size of thefoams is. In the present invention, the nucleating agent of 0.1 to 0.4%based on the weight of a resin is used. When the amount of the usednucleating agent exceeds 0.4%, insufficient dispersion or agglomerationcan occur, and as a result, the cell grows larger than a predeterminedsize. On the contrary, when the amount of the used nucleating agent is0.1% or less, the nucleation activity is excessively weak, whereby thecell diameter can not be reduced.

As a usable foaming agent in the present invention, there are organicfoaming agents and inorganic foaming agents. Examples of the organicfoaming agent are aliphatic hydrocarbons such as propane, butane, hexaneand heptane, alicyclic hydrocarbons such as cyclobutane andcyclopentane, and halogenated hydrocarbons such as chlorofluorometnae,trifluorometane, 1,1-difluoroetnae, 1,2,2,2-tetrafluoroetane, methylchloride, ethyl chloride and methylene chloride. Also, usable organicfoaming agents include dichlorotetrafluoroethane,trichlorotrifluoroethane, trichloromonofluoromethane,dichlorodifluoromethane, dichloromonofluoromethane anddibromotetrafluoroethane. Considering forming workability, nontoxicityand flame retardancy, these fluoro-chlorinated hydroncarbon arepreferable. These organic foaming agents can be used alone or as amixture of two or more thereof. Examples of the inorganic foaming agentinclude nitrogen, carbon dioxide, argon and air. These inorganic foamingagents can be used alone or as a mixture of two or more thereof. Also,any mixtures of randomly selected two or more of the organic foamingagents and the inorganic foaming argents can be used. The most preferredfoaming argent is an inorganic foaming agent since they do not destroyan ozonosphere and are inexpensive. The used amount of the foamingagents depends upon the expansion ratio of the foam pellet to beobtained and the type of the used resin and foaming agent. The amount ofthe foaming agent used in the present invention is about 0.1% to 0.4% byweight, based on the weight of the resin.

In addition, various kinds of additives can be used. Examples of suchadditives include an antioxidant, UV absorber, flame retardant, coloringagent, dye, metal deactivator and the like. These additives can be usedin an amount of 0.1 to 0.3% by weight, based on the weight of thecopolymer resin. In a preferred embodiment of the present invention,paraffin wax is used. This helps the copolymer resin to flow and acts asan antistatic agent to remove static electricity of the copolymer resin.The used amount of paraffin wax is about 0.1% by weight.

The above materials used in the present invention are extruded by thetandem extruder in which a specific physical condition is set accordingto the present invention and the process will be described as follows.The tandem extruder is consisted of a first extruder, a second extruderand a guide connecting the first and second extruders. A screwcompression ratio is generally 3:1. Usually, the inner diameter of acylinder of the first extruder is about 60 to 70 mm. The inner diameterof a cylinder of the second extruder is normally about 90 to 95 mm.

The first extruder is divided into six zones according to theirtemperatures, each zone corresponds to 300 to 400 LD. The six zones area first temperature zone of 148 to 153° C., a second temperature zone of167 to 172° C., a third temperature zone of 167 to 172° C., a fourthtemperature zone of 218 to 223° C., a fifth temperature zone of 197 to203° C., and a sixth temperature zone of 188 to 193° C.

The non-crosslinked polypropylene random copolymer resin and thenucleating agent are supplied at a constant speed through a hopper andthen melted in the first temperature zone in which a temperature of 148to 153° C. is set. The flowing speed can be adjusted by a rotating speedof the screw and is normally about 20 to 30 rpm. Such rotating speed ofthe screw determines an inflow speed of row material and a flow speed ofmelt. In this case, an inflow speed of the resin is about 25 km/hour.Though the copolymer resin and the nucleating agent can be suppliedthrough one hopper at the same time, it is preferable that they aresupplied independently via individual hoppers. The second and thirdtemperature zones are maintained at a temperature of 167 to 172° C.,other additives such as paraffin wax are introduced to an starting pointof the third temperature zone. The paraffin wax is pumped to beintroduced after it is melted. Also, the temperature of the fourthtemperature zone is set to 218 to 223° C. and a foaming agent issupplied to an starting point of the fourth temperature zone by pumpingoperation. The melt from the fourth temperature zone is passed throughthe fifth temperature zone in which a temperature of 218 to 223° C. isset and then introduced to the sixth temperature zone in which atemperature of 188 to 193° C. is set.

The melt from the sixth temperature zone of the first extruder isintroduced to the second extruder through the guide connecting the firstand second extruders. Here, the temperature of the guide is set to 248to 255° C. and LD of the guide is 300 to 400 mm.

The second extruder is also divided into six zones according to atemperature, each zone corresponds to 470 to 520 mm. The six zones are afirst temperature zone of 168 to 173° C., a second temperature zone of147 to 152° C., a third temperature zone of 143 to 147° C., a fourthtemperature zone of 137 to 142° C., a fifth temperature zone of 137 to142° C., and a sixth temperature zone of 132 to 137° C. A screw speed ofthe second extruder is normally 8 to 12 rpm.

Each zone in the first and second extruders can be maintained at aspecifically set temperature by means of any one of water-cooled type,oil-cooled type and air-cooled type method. Among them, the water-cooledmethod that adjusts a temperature using water pressure is preferable.For example, a water cooled type device can be used, which has a coolingwater body which is formed in a hollow shape, so that it can be mountedaround the cylinder of the extruder and the cylinder is insertedtherein, and is provided with a cooling water passage formed integrallywith the body, so that cooling water is contacted directly with thesurface of the cylinder.

(2) Pumping

The sixth temperature zone of the second extruder can correspond to aflange for connecting a means for crushing the melt according to amethod of the present invention. Since the temperature of this flange ismaintained peculiarly at 132 to 137° C. by the present invention andthis temperature is lower than the melting point of 138° C. of thepolypropylene resin, an flowing speed of the melt can be remarkablylowered. Thus, there is need for forcibly flowing the melt so that itcan move smoothly. Such compulsory flow can be achieved by means of apump. At this time, the temperature is maintained at 125 to 138° C. bythe water-cooled type device.

(3) Homogenizing

According to the present invention, the melt transferred from theextruder by pumping at a temperature of 125 to 140° C. is homogenized ata temperature of 120 to 130° C. Here, to homogenize means that the meltis cut and ground at the same time in a manner of grinding stones. Also,through homogenization, the melt becomes to have a uniform temperaturein the inner portion and outer portion. In the course of homogenization,the temperature in the cylinder is kept at 120 to 130° C., preferably bya water cooling type or oil cooling type method, more preferably oilcooling type method. At this time, the pressure inside the cylinderreaches about 120 kg/cm².

(4) Expanding

The homogenized melt is expanded through dies. As described above, sincea pressure inside the cylinder in which the melt are homogenized reachesabout 120 kgf/cm², a decompression means is installed at the dies tomaintain a pressure of 0.3 to 0.7 kgf/cm². The polypropylene resin isexpanded through holes of the dies. Here, a diameter of each hole isnormally 0.5 to 1.0 mm, and an expansion ratio is normally five times ofa diameter of the micro hole.

(5) Pelleting

The foams formed by expansion through the holes of the dies aredischarged and simultaneously cut by cutting members to produce pellettype foams.

A device for producing the pellet-type foams of non-crosslinkedpolypropylene which have a melting point of 125 to 140° C. according tothe present invention comprises a first extruder, a second extruderconnected to the first extruder, a pumping part connected to the secondextruder, a homogenizing part connected to the pumping part and a diespart connected to the homogenizing part

The first extruder comprises a cylindrical cylinder having a screw shaftrotatably mounted therein, a driving means disposed at an end of thecylinder for rotating the screw shaft and a plurality of cooling meansand heating means disposed on an the circumference surface of thecylinder and is provided with inlets for supplying a polypropylene and anucleating agent to the cylinder in an end portion of the cylinder nearthe driving means is and inlets for supplying an additive such as anantistatic agent and a foaming agent in a proper middle portion of thecylinder and an outlet at the other end portion of the cylinder. Thepolypropylene and a nucleating agent supplied to the cylinder via theinlet are transfered compulsorily toward the outlet by the screw shaftwhich is rotated by the driving means.

The second extruder connected to the first extruder by a guide comprisesa cylinder to which the polypropylene melt discharged from the cylinderof the first extruder is supplied through the guide, and a plurality ofcooling means and a heating means mounted to the outer circumferencesurface of the cylinder for adjusting a temperature of the melt in thecylinder.

The pumping part for moving compulsorily the polypropylene meltdischarged from the second extruder to the next device comprises acasing having an inner space to which the polypropylene melt dischargedfrom the cylinder of the second extruder is supplied, a pair of gearsrotatably installed in the casing, the gears being engaged with eachother, and a driving means for rotating the gears.

The homogenizing part comprises a cylindrical first housing to which thepolypropylene melt discharged from the cylinder of the pumping part issupplied, the first housing being rotatably installed rotatably, adriving means for rotating the first housing, a screw connected to adischarging end of the first housing, a second housing located on acircumference part of the screw, a frame mounted to an outercircumference of the second housing for forming an airtight spacebetween the second housing and the frame. A spiral space is formedbetween the screw and the second housing along the entire length of thescrew and the polypropylene melt discharged from the first housing isflow therethrough to be discharged and discharged to the outside. A heattransfer fluid oil flows in a space formed between the second housingand the frame to adjust a temperature of the polypropylene melt whichflows through the second housing.

The homogenizing part comprises a homogenizing means that crushesuniformly the polypropylene melt. The homogenizing means is composed ofa rotating plate rotatably mounted and a fixing plate disposed to becontacted with the rotating plate. The rotating plate is provided with aplurality of slits arranged radially and the fixing plate is providedwith a plurality of circular holes. The polypropylene melt introduced tothe homogenizing part arrives at the rotating plate and is cut by anedge of each opening of the rotating plate while it passes through therotating plate. Then, the cut polypropylene melt is ground by therotating plate in the space between the rotating plate and the fixingplate.

The ground polypropylene melt discharged from the homogenizing part issupplied to the dies part including a discharging part, a cutting partand a driving means, in which the expanded foams are cut into apredetermined dimension.

In the present invention, the cooling means mounted to the cylinders ofthe first or second extruder has a closed casing through which coolingwater supplied from the outside flows. The cooling water introduced tothe inside of the casing flows through the casing while being in contactwith the surface of the cylinder whereby the temperature of thepolypropylene melt that flows within the cylinder is reduced. Theheating means disposed between two casings employs a heater having aheating coil installed therein.

The cylinder of the first extruder is divided into six temperature zonesaccording to a temperature condition which the polypropylene meltflowing in the cylinder should satisfy. A temperature of each zone isadjusted by the cooling means or heating means mounted on the outercircumference of the cylinder. A temperature of the polypropylene meltshould be kept at 147 to 153° C. in the first temperature zone, at 167to 172° C. in the second temperature zone, at 168 to 172° C. in thethird temperature zone, at 218 to 225° C. in the fourth temperaturezone, at 197 to 203° C. in the fifth temperature zone and at 188 to 193°C. in the sixth temperature zone.

The cylinder of the second extruder is also divided into six temperaturezones according to a temperature condition which the polypropylene meltflowing in the cylinder should satisfy. A temperature of each zone isadjusted by the cooling means or heating means mounted on an outercircumference of the cylinder. A temperature of the polypropylene meltshould be at 167 to 173° C. in the first temperature zone, at 147 to152° C. in the second temperature zone, at 142 to 147° C. in the thirdtemperature zone, at 137 to 141° C. in the fourth temperature zone, at137 to 142° C. in the fifth temperature zone and at 132 to 137° C. inthe sixth temperature zone.

The guide connecting the first extruder and the second extruder shouldbe kept at a temperature of 248 to 255° C.

The two gears disposed in the inner space of the casing of the pumpingpart rotate in opposite directions from each other toward a center ofthe inner space to make the polypropylene melt move compulsorily to anext process position. Also, the first housing of the homogenizing partis rotatably supported on supporting plates by a plurality of bearingblocks. A drive sprocket of a driving means is geared with a drivensprocket fixed to an outer circumference surface of the first housing sothat the first housing is rotated in response to the operation of thedriving means.

The discharging part of the dies part includes a hollow guide bar and acylinder located outside of the guide bar. The guide bar has a pluralityof cavities formed on the outer circumference thereof in a longitudinaldirection of the guide bar. The polypropylene melts discharged from thehomogenizing part flows through each cavity. A plurality of throughholes are formed at portions of the cylinder corresponding to thecavities, respectively.

The cutting part of the dies part includes a supporting plate located ata rear side of the discharging part and cutting member fixed to thesupporting plate and movably located outside of the cylinder. Thecutting member is provided with a plurality of through holes formed atpositions corresponding to the plurality of through holes on thecylinder, respectively. The driving means comprises an eccentric cambeing able to rotate by a motor, a crank connected to the eccentric camand rotating in response to the rotation of the cam, and a powerconverting and transmitting means connected to the crank for convertinga rotating movement of the crank into a linear movement and transmittingthe linear movement to the supporting plate to which the cutting memberis fixed, whereby the cutting members are reciprocated along the outercircumference of the cylinder by the operation of the driving means andthe foams expanded from the through holes of the cylinder are cut byedges of the through holes of the cutting member.

Meanwhile, the cylinder provided with a plurality of grooves formed in alongitudinal direction on predetermined position thereof. Each groovehas a reciprocating rod which is movably located therein and has one endfixed to the supporting plate. A cutting member is fixed to eachreciprocating rod by a fixing means and is reciprocated on an outercircumference of the cylinder by the reciprocating rod which isreciprocated along each groove of the cylinder.

Now, a particular structure and operation of the device for producingpellet-type foams according to the present invention will be describedin more detail taken in conjunction with the accompanying drawings.

FIG. 1 is a view showing an overall structure of a device for producingthe pellet-type foams according to the present invention. The device forproducing foams according to the present invention comprises the firstextruder 100, the second extruder 200 connected to the first extruder100 via a guide 150, a pumping part 300 connected to the second extruder200, a homogenizing part 400 connected to the pumping part 300 and adies part 500 for forming a foam mixture discharged from thehomogenizing part 400 into pellet-type foams. Hereinafter, respectiveparts as mentioned above will be described individually.

A. First extruder 100.

FIG. 2 is a plan view showing a cylinder of the first extruder 100 shownin FIG. 1 and FIG. 3 is a sectional view taken along the line A—A inFIG. 2. These figures show a structure of the first extruder 100. Thefirst extruder 100 comprises a cylindrical cylinder 101 having a certainlength, a driving means 102 installed at an end of the cylinder 101, ascrew shaft 103 mounted in the cylinder 101 and being able to rotate bythe driving means 102, and a cooling means 104 and heating means 105installed on the outer circumference surface of the cylinder 101.

At an end portion of the cylinder 101 near the driving means 102,entrances 101 a, 101 b (only one entrance 101 a is shown in FIG. 3 whichis a sectional view) for supplying a row polypropylene and a nucleatingagent (for example, sodium bicarbonate) to the cylinder 101,respectively, are formed. An exit 101 c is formed at another end portionof the cylinder 101. Also, at a middle portion of the cylinder 101, anentrance 101 d for supplying an antistatic agent (for example, paraffinwax) and an entrance 101 e for supplying a foaming agent (for example,LPG or CO₂) are formed.

On the other hand, an interior of the cylinder 101 is divided into sixtemperature zones according to temperature conditions of thepolypropylene that is supplied to the cylinder 101. The cooling means104 and the heating means 105 are mounted to the outer circumferencesurface of the cylinder 101 at positions corresponding to respectivetemperature zones for adjusting the temperature of the polypropylenemelt.

FIG. 2 is a plan view showing the cooling means 104 and the heatingmeans 105 mounted on the outer circumference surface of the cylinder 101of the first extruder 100 shown in FIG. 1. Now, the cooling means andheating means will be explained in detail referring to FIG. 3.

The cooling means 104 mounted on the outer circumference surface of thecylinder 101 corresponding to each zone has an airtight casing with adonut shape, through which a cooling water supplied from an outsideflows. The cooling water introduced to the casing 104 flows through thecasing 104 while being in contact with the surface of the cylinder 101.Therefore, the temperature inside the cylinder 101, that is, thetemperature of the polypropylene melt that flows therein can be lowered.

The heating means 105 mounted between two casings 104 (that is, coolingmeans) is a heater in which a heating coil is installed. The heatingmeans raise the temperature of the polypropylene melt which has beenalready lowered by the cooling means 104 to a predetermined temperature.

The function of the first extruder 100 having a structure as describedabove will be described with reference to respective drawings. As thedriving means 102 is activated, polypropylene and a nucleating agent aresupplied to the cylinder 101 through the entrances 110 a, 101 b,respectively. The screw shaft 103 is rotated in the cylinder 101 by theaction of the driving means 102 (of course, a rotating speed of thescrew shaft is lowered by a speed reducer, as compared with a rotatingspeed of the driving means), whereby the polypropylene and thenucleating agent supplied to the cylinder 101 are melted and mixed whilesimultaneously being moved compulsorily toward the other end of thecylinder 101.

In the process as described above, an antistatic agent and a foamingagent are supplied to the cylinder 101 through another entrances 101 d,101 e formed at a mid portion of the cylinder 101 to be mixed with apolypropylene melt.

As described above, the cylinder 101 of the first extruder 100 isdivided into six temperature zones Z1 to Z6 according to the temperaturecondition of the polypropylene melt which flows therein as shown in FIG.2. Each of the temperature zones Z1 to Z6 has a length of about 300 to400 mm. In a preferred aspect, the cylinder 101 has an inner diameter of65 mm and LD of about 358 mm and the temperature condition and otherconditions of the polypropylene melt according to each temperature zoneZ1 to Z6 of the cylinder are as follows.

-   -   1) First temperature zone Z1: A zone to which the polypropylene        and the nucleating agent are supplied, kept at a temperature of        150° C., a length for which the above temperature is maintained,        that is, a length of the first temperature zone Z1 is 360 mm.    -   2) Second temperature zone Z2: The polypropylene melt flowing        through the second temperature zone Z2 is maintained at a        temperature of 170° C.    -   3) Third temperature zone Z3: The polypropylene melt flowing        through the third temperature zone Z3 is maintained at a        temperature of 170° C. which is the same as in the second        temperature zone Z2. Paraffin wax as an antistatic agent is        supplied to the third temperature zone Z3.    -   4) Fourth temperature zone Z4: The polypropylene melt flowing        through the fourth temperature zone Z4 is maintained at a        temperature of 220° C. CO₂ or LPG as a foaming agent is supplied        to the cylinder 101 at the fourth temperature zone Z4.    -   5) Fifth temperature zone Z5: The polypropylene melt flowing        through the fifth temperature zone Z5 is maintained at a        temperature of 200° C.    -   6) Sixth temperature zone Z6: The polypropylene melt is        maintained at a temperature of 190° C.

In order to meet the temperature conditions of the polypropylene melt atrespective temperature zones Z1 to Z6, the cooling means 104 and theheating means 105 mounted on the temperature zones are properlyoperated. That is, the temperatures of the polypropylene melt at therespective temperature zones Z1 to Z6 are adjusted to meet the conditiondescribed above by controlling an amount and a temperature of a coolantsupplied to the casing of the cooling means 104 or a current applied tothe heating wire constituting the heating means 105 and a time to applya current.

On the other hand, in the case of using CO₂ as a foaming agent which issupplied to the cylinder 101 of the first extruder 100, an additionaldevice for supplying CO₂ is used. The device for supplying CO₂ used inthe present invention is as follow:

Referring to FIG. 4 which is a schematic view showing a structure of thedevice for supplying CO₂, the device 110 for supplying CO₂ comprises atank 110A for storing CO₂, an unit for vaporizing and freezing 110B, anunit for supplying CO₂ 110C, an unit for stabilizing 110D and a storageunit 110E. CO₂ stored in the storing CO₂ tank 110A is transferred to theunit 110B for vaporizing and freezing, in which it is converted into thevapor phase. That is, in the course of passing through a refrigerator ofthe unit for vaporizing and freezing 110B, CO₂ is gasified andvaporized, and then supplied to the unit 110D for stabilizing by a pump,the unit 110C for supplying CO₂. In the unit 110D for stabilizing, CO₂in the form of vapor is converted to the gas phase. CO₂ in the gas phaseis stored in the storage unit 110E. When a process is started, CO₂stored in the storage unit 110E is supplied to the first cylinder 101 ofthe first extruder 100 described above.

The polypropylene melt which meets the temperature conditions in thetemperature zones Z1 to Z6 is moved (moved by the screw shaft 103)toward an end of the cylinder 101 and then supplied to the secondextruder 200 through the guide 150. The polypropylene melt passingthrough the guide 150 is maintained at a temperature of 250° C.

B. Second extruder 200

The second extruder 200 to which the polypropylene melt is suppliedthrough the guide 150 has the same structure with the first extruder100. That is, the cylinder 201 constituting the second extruder 200 isdivided into six temperature zones according to the temperaturecondition of the polypropylene melt that flows in the cylinder 201. Acooling means and a heating means are mounted on the outer circumferencesurface of the cylinder 201 at positions corresponding to thetemperature zone for adjusting the temperature of the polypropylenemelt.

The cooling means and heating means also have the same structure withthe cooling means 104 and the heating means 105 mounted on the outercircumference surface of the cylinder 101 of the first extruder 100shown in FIG. 2. Therefore, the description on the structures of thecooling means and heating means is omitted.

The cylinder 201 of the second extruder 200 is divided into sixtemperature zones according to the temperature condition of thepolypropylene melt which flows therein. Each temperature zone has alength of about 470 to 520 mm. In a preferred aspect, the cylinder 201has an inner diameter of 90 mm and LD is about 495 mm. The temperatureconditions of the polypropylene melt in the cylinder 201 at thetemperature zones are as follows.

1) First temperature zone (the entrance portlion): 170° C.

2) Second temperature zone: 150° C.

3) Third temperature zone: 145° C.

4) Fourth and fifth temperature zones: 140° C.

5) Sixth temperature zone (the exit portion): 135° C.

C. Pumping part 300

The polypropylene melt at a temperature of 135° C. discharged from thesecond extruder 200 is supplied to the pumping part 300. Since themelting point of the polypropylene is 138° C., the polypropylene meltdischarged from the second extruder 200 has been considerably reduced inits flowing speed. In the present invention, the pumping part 300 isused for moving compulsorily such polypropylene melt to a subsequentprocess.

FIG. 5 is a sectional view showing the inner structure of the pumpingpart 300 and homogenizing part 400 shown in FIG. 1. The left side showsthe inner structure of the pumping part 300 and the right side shows aninner structure of the homogenizing part 400, respectively.

The pumping part 300 to which the polypropylene melt discharged from thecylinder 201 of the second extruder 200 is supplied comprises a casing301 having an inner space, a pair of gears 302A and 302B engaged witheach other and installed rotatably in the inner space of the casing 301,and a driving means 303 for rotating the gears 302A and 302B. Anentrance portion of the casing 301 is connected to the cylinder 201 ofthe second extruder 200 by a flange 304.

The polypropylene melt discharged from the cylinder 201 of the secondextruder 200 is supplied to the inner space of the casing 301 via theentrance portion, and then discharged compulsorily to an exit portion bythe two gears 302A and 302B which rotate in opposite directions toward acenter of the inner space of the casing. The polypropylene meltdischarged from the cylinder 201 of the second extruder 200 has atemperature of 135° C., and a considerably reduced fluidity. Therefore,the pumping part 300 serves to compulsorily transfer the polypropylenemelt to the next process.

D. Homogenizing part 400

The homogenizing part 400 connected to the exit portion of the casing301 of the pumping part 300 is divided into a rotating part 400A and acrushing part 400B. The rotating part 400A is composed of a firsthousing 401 of a hollow cylindrical shape, a driven sprocket 402 fixedto the outer surface of the first housing 401 and a driving means 404 towhich a driving sprocket 403 is fixed.

The first housing 401 is supported rotatably to support plates 407 and408 through a plurality of bearing blocks 405 and 406. The drivesprocket 403 of the driving means 404 is geared with the driven sprocket402 fixed to the outer surface of the first housing 401, whereby thefirst housing 401 is rotated in response to the operation of the drivingmeans 404. An end of the first housing 401 corresponds to the exitportion of the casing 301 of the pumping part 300 and thus, thepolypropylene melt discharged from the pumping part 300 flows in thefirst housing 401. In the first housing 401, the temperature of thepolypropylene melt are varied according to the location (that is, acentral portion and outer portion of the inner space of the firsthousing). However, since the polypropylene melt is mixed by the rotationmovement of the first housing 401, the whole polypropylene melt ismaintained constantly at a temperature. Here, since new polypropylenemelt is supplied compulsorily and continuously by the pumping part 400,the polypropylene melt is rotated and moved at the same time.

The crushing part 400B is composed of a screw 410 connected to the exitportion of the first housing 401, a second housing 411 located on theouter circumference of the screw 410 and a frame 412 mounted on theouter circumference of the second housing 411 for forming an airtightspace between the second housing 411 and the frame 412. The screw 410 isa cylindrical member having a spiral with a certain depth formed on theouter circumference surface thereof and a spiral-shaped space is formedbetween the screw 410 and the second housing 411 and extended to theentire length of the screw 410. Therefore, the polypropylene meltdischarged from the first housing 401 of the rotating part 400A under ahigh pressure, is moved along the spiral-shaped space between the screw410 and the second housing 411 and discharged out of the crushing part400B.

Meanwhile, a heat transfer oil flows in the space formed between thesecond housing 411 and the frame 412. That is, an inlet port 412Athrough which the heat transfer oil is supplied is formed at a side ofthe frame 412, an outlet 412B through which the heat transfer oil isdischarged is formed at the otner side of the frame 412. The heattransfer oil that introduced to the space between the second housing 411and the frame 412 via the inlet port 412A contacts directly with thesurface of the second housing 411 to adjust the temperature of thepolypropylene melt that flows in the second housing 411. The heattransfer oil that flows in the space between the second housing 411 andthe frame 412 to adjust the temperature of the polypropylene melt isdischarged via the outlet port 412B. Processes for inflow, adjustment oftemperature and discharge are proceeded continuously, whereby thetemperature of the polypropylene melt which flows in the spiral-shapedspace between the second housing 411 and the screw 410 is adjusted to apredetermined temperature.

At the rear end of the homogenizing part 400, that is, the exit part ofthe second housing 411, a homogenizing means 450 is disposed forcrushing and homogenizing a mixture of the discharged polypropylene meltand a nucleating agent.

FIG. 6A and FIG. 6B are front views of a rotating plate 451 and a fixingplate 452 constituting the homogenizing means 450, respectively. Therotating plate 451 and the fixing plate 452 have same shape and aremounted in a state that they are contacted with each other, providedthat the rotating plate 451 is mounted rotatably.

A plurality of openings 451A are formed radially on the rotating plate451, each opening 451A is slanted toward a center of the rotating plate451. Also, a plurality of circular holes 452A are formed on the fixingplate 452.

The polypropylene melt discharged from the second cylinder 411 arrivesat the rotating plate 451 which is rotating and passes through eachopening 451A formed on the rotating plate 451. At this point, thepolypropylene melt is cut by an edge of each opening 451A, whereby allpolypropylene melts are crushed homogeneously. The crushed polypropylenemelt is ground by the rotating plate 451 which rotates in the spacebetween the rotating plate 451 and the fixing plate 452 and dischargedthrough the holes 452A of the fixing plate 452.

E. Dies part 500

The temperature-controlled polypropylene melts discharged from thehomogenizing part 400 is supplied to the dies part 500 to producepellet-type foams.

FIG. 7 is a plan view showing a structure of the dies part shown in FIG.1 and FIG. 8 is a sectional view taken along the line B—B in FIG. 7, andFIG. 9 is a sectional view taken along the line C—C in FIG. 8. The diespart 500 is divided into a discharging part 500A, a cutting part 500Band a driving means 500C.

The discharging part 500A is composed of a hollow cylinder-shaped guidebar 501 and a cylinder 502 located outside the guide bar 501. On theouter circumference of the guide bar 501, a plurality of cavities 501Aare formed in a longitudinal direction of the guide bar 501. Thepolypropylene melt discharged from the homogenizing part 400 flows ineach cavity 501A. The plurality of through holes 502A are formed atportions of the cylinder 502 corresponding to the respective cavities501A.

The cutting part 500B is composed of a supporting plate 506 located atthe rear side of the discharging part 500A and a cutting member 503fixed to the supporting plate 506. The cutting member 503 is locatedmovably on the outside of the cylinder 502 and has a plurality ofthrough holes 503A formed at positions corresponding to the plurality ofthrough holes 502A of the cylinder 502, as shown in FIG. 9. As shown infigures, a diameter of the through holes 502A formed on the cylinder 502is less than that of the through holes 503A formed on the cutting member503. At an initial position, each through hole 502A of the cylinder 502is located on a central portion of the corresponding through hole 503Aof the cutting member 503.

Meanwhile, a plurality of grooves 502B are formed on the outercircumference of the cylinder 502 in a longitudinal direction andreciprocating rods 505 are located in the grooves 502B, respectively.The cutting member 503 is fixed to reciprocating rods 505 by fixingmeans such as a bolt, whereby the cutting member 503 is reciprocated onthe outer circumference of the cylinder 502 by the reciprocating rod 505which is reciprocated along the groove 502B of the cylinder 502.

The driving means 500C of the dies part 500 is composed of an eccentriccam 511 which is rotated by a motor 510, a crank 512 that is connectedto the eccentric cam 511 and rotated in response to the rotation of thecam 511 and a power converting and transmitting means 513 connected tothe crank 512 for converting rotation movement of the crank 512 tolinear movement and transmitting the linear movement to the supportingplate 506.

Once the eccentric cam 511 is rotated by the operation of the motor 510,the rotation movement of the crank 512 is converted to the linearmovement by the power converting and transmitting means 513, and thentransmitted to the supporting plate 506 to which ends of reciprocatingrods 505 are fixed. Therefore, the cutting member 503 is reciprocatedalong the outer circumference of the cylinder 502.

Meanwhile, the polypropylene melt discharged from the homogenizing part400 is injected under pressure to the plurality of cavities 501A formedon the guide bar 501 and then expanded through the through holes 502A ofthe cylinder 502. At this time, when each through hole 502A of thecylinder 502 corresponds to each through hole 503A of the cutting member503 by the reciprocation of the cutting member 503 by the operation ofthe driving means 510, the polypropylene melt passes through the throughholes 502A and 503A and then is expanded to the outside of the cuttingmember 503 with a certain length. Then, when through holes 502A of thecylinder 502 are released from the corresponding state with the throughholes 503A of the cutting member 503 by the movement of the cuttingmember 503, the expanded polypropylene melt is cut by edges of thethrough holes 503A of the cutting member 503. Here, a dimension (length)of the cut foams is determined by a moving speed of the cutting members503.

Thus, after the polypropylene melt is expanding through the throughholes 502A of the cylinder 502 and cut by the cutting member 503, thepellet-type foams are formed. A diameter of each through hole 502A ofthe cylinder 502 is 0.7 mm and the expansion rate is about 5 times of adiameter of the through hole. Also, the cutting member 503 isreciprocated at a speed of 600 revolutions per minute.

The polypropylene melt transferred from the crushing part 400B aresubjected under a pressure of 120 kgf/cm² in the dies part. If thepolypropylene melt in the dies is exposed directly to the atmosphere,most of the foams are open cells. In order to prevent production of opencells, in the present invention, a decompression means is installed atthe outside of the dies part.

In FIG. 7, an example of the decompression means installed at the diespart is shown. The decompression means 600 is a casing 601 for isolatingthe discharging part 500A and the cutting part 500B of the dies part 500from the outside (the atmosphere). It is to be understood that a shapeof the casing 601 is not limited. An entrance port 602 through which airis introduced is formed at an side of the casing 601, an exit port 603through which air is exhausted is formed at the other side of the casing601.

A temperature of the air which is supplied to the casing 601 is roomtemperature or less and can be maintained by a cooling means (notshown). Also, it goes without saying that, in order to maintain properlya pressure in the casing 601 (for example, 0.8 Kgf/cm²), an amount ofthe air supplied to the casing 601 can be controlled by a pump (notshown). Meanwhile, the exit port 603 can be connected to a storage meansfor storing the prepared pellet-type foams along with discharged air.

A temperature in the cylinder 502 constituting the dies part 500 is veryhigh and therefore, the temperature of the cylinder 502 should beproperly maintained. For this, the present invention uses a coolingdevice 700 using the heat transfer oil, which is mounted on the diespart.

FIG. 10A is a side view showing a cylinder of the dies part to which acooling device is mounted and FIG. 10B is the front view of FIG. 10A.The casing 601 of the decompression means 600 illustrated in FIG. 7 isnot shown for conveniences' sake.

The cooling device 700 used in the present invention comprises aring-type supplying pipe 701 located at the front of the cylinder 502,and through which heat transfer oil is supplied from the outside, aplurality of flowing pipes 702 connected to the supplying pipe 701 viaan entrance end 702A thereof and installed in the cylinder 502, and andischarging pipe 703 located at the front of the cylinder 502 andconnected to an exit end 702B of the flowing pipe 702.

The ring-type pipe 701 for supplying the heat transfer oil, located atthe front of the cylinder 502, is connected at one end to a compulsoryflowing means (not shown) such as a pump so that the heat transfer oilis supplied to the supplying pipe at a constant pressure. The heattransfer oil is injected to the plurality of flowing pipes 702 throughentrance ends 702A of the flowing pipes 702 connected to the supplyingpipe 701.

The plurality of the flowing pipes 702 disposed at a regular interval onthe entire outer circumference of the cylinder 502 are extended alongthe entire length of the cylinder 502, and each entrance end 702A andeach exit end 702B of the flowing pipes 702 are exposed to the front endof the cylinder 502. Therefore, the heat transfer oil supplied througheach entrance end 702A from the heat transfer oil supplying pipe 701flows through the flowing pipes 702 along the entire length of thecylinder 502 (that is, after performing the heat exchange), and then isdischarged via each exit end 702B.

The ring-type pipe 703 for discharging the heat transfer oil located atthe front of the cylinder 502 is connected at one end to the heattransfer oil storage tank (not shown). Thus, the heat transfer oil afterperforming heat exchange with the cylinder 502 while flowing through theflowing pipes 702 flows into the heat transfer oil discharging pipe 703through the exit end 702B and then is directed to the heat transfer oilstorage tank.

As described above, in the course of flowing through the heat transferoil supplying pipe 701, the flowing pipes 702 installed in the cylinder702 and the heat transfer oil discharging pipe 703, the heat exchangebetween the heat transfer oil and the cylinder 502 is accomplished,whereby the cylinder can be constantly Kept at a suitable temperaturefor the production of foams.

Meanwhile, the supplying pipe 701 for supplying the heat transfer oil tothe flowing pipes 702 and the discharging pipe 703 to which the heattransfer oil from the flowing pipes 702 is supplied have a ring shape sothat the heat transfer oil is simultaneously supplied to and receivedfrom the flowing pipes 702 mounted on the circumference of the cylinder502 whose section is a circular shape. However, the shape of thesupplying pipe 701 and the discharging pipe 703 is not limited to a ringand can have a different shape, for example polygonal shape.

Now, the present invention will be described in more detail by thefollowing Examples. However, it is to be understood that the followingexamples are presented to illustrate further various aspects of thepresent invention, but are not intended to limit the scope of theinvention in any aspects.

EXAMPLE 1

In order to prepare the pellet-type foams of non-crosslinkedpolypropylene according to the invention, a tandem extruder having afirst extruder with the inner diameter of 65 mm and a second extruderwith the inner diameter of 900 mm was modified. A gear pump and a diespart were connected successively to the rear end of the second extruder.A homogenizing means as shown in FIG. 3 was installed at a positionwhere the melt is discharged from the gear pump, a compression means isprovided outside the dies. Through holes of the dies had a diameter of0.7 mm and a pressure in the dies was maintained at 0.5 kgf/cm². Arotating speed of the first extruder was set at 24 rpm and a rotatingspeed of the second extruder was set at 9 rpm.

40 kg of random copolymer RP2400 (polypropylene-3 weight % ethylene;melting index 0.25; melting point 138° C.) commercially obtained fromYuhwa Korea Petrochemical Ind. Co., Ltd. and 800 g of sodium carbonatecommercially obtained from Keum Yang Co., Ltd. were supplied to theextruder through respective hoppers. 300 g of paraffin wax M1commercially obtained from Leochemical Co., Ltd (Kimhae, Korea) wassupplied to the third temperature zone of the first extruder. 12 kg ofLPG was supplied to the fourth temperature of the extruder by using ametering pump. Temperature conditions specified from the extruderthrough the homogenizing device are shown in Table 1, and LD of the eachtemperature zone was 360 mm.

TABLE 1 Temperature condition Temperature Temperature Device Zone No. (°C.) First 1^(st) 150 Extruder 2^(nd) 170 3^(rd) 170 4^(th) 220 5^(th)200 6^(th) 190 Guide* 250 Second 1^(st) 170 Extruder 2^(nd) 150 3^(rd)144 4^(th) 139 5^(th) 138 6^(th) 136 Gear pump 133 Homogenizing device130 *installed between the first extruder and second extruder forguiding the melt that have passed from the sixth temperature zone of thefirst extruder to the first temperature zone of the second extruder.

EXAMPLE 2

The temperature conditions in the device were set as described in Table2 and the pellet-type foams were prepared by using the same procedureand materials as in Example 1.

TABLE 2 Temperature condition Temperature Temperature Device Zone No. (°C.) First 1^(st) 150 Extruder 2^(nd) 170 3^(rd) 170 4^(th) 220 5^(th)200 6^(th) 190 Guide 250 Second 1^(st) 170 Extruder 2^(nd) 150 3^(rd)145 4^(th) 140 5^(th) 138 6^(th) 135 Gear pump 130 Homogenizing device125

EXAMPLE 3

The temperature conditions in the device were set as described in Table3 and the pellet-type foams were prepared by using the same procedureand materials as in Example 1.

TABLE 3 Temperature condition Temperature Temperature Device Zone No. (°C.) First 1^(st) 147 Extruder 2^(nd) 167 3^(rd) 168 4^(th) 218 5^(th)202 6^(th) 188 Guide 248 Second 1^(st) 167 Extruder 2^(nd) 147 3^(rd)142 4^(th) 137 5^(th) 137 6^(th) 132 Gear pump 130 Homogenizing device129

EXAMPLE 4

The temperature conditions in the device were set as described in Table4 and the pellet-type foams were prepared by using the same procedureand materials as in Example 1.

TABLE 4 Temperature condition Temperature Temperature Device Zone No. (°C.) First 1^(st) 151 Extruder 2^(nd) 170 3^(rd) 170 4^(th) 219 5^(th)202 6^(th) 190 Guide 252 Second 1^(st) 170 Extruder 2^(nd) 150 3^(rd)146 4^(th) 141 5^(th) 140 6^(th) 135 Gear pump 130 Homogenizing device127

EXAMPLE 5

The temperature conditions in the device were set as described in Table5 and the pellet-type foams were prepared by using the same procedureand materials as in Example 1.

TABLE 5 Temperature condition Temperature Temperature Device Zone No. (°C.) First 1^(st) 153 Extruder 2^(nd) 172 3^(rd) 172 4^(th) 225 5^(th)203 6^(th) 193 Guide 255 Second 1^(st) 173 Extruder 2^(nd) 152 3^(rd)147 4^(th) 141 5^(th) 142 6^(th) 137 Gear pump 134 Homogenizing device130

EXAMPLE 6

The temperature conditions in the device were set as described in Table6 and the pellet-type foams were prepared by using the same procedureand materials as in Example 1.

TABLE 6 Temperature condition Temperature Temperature Device Zone No. (°C.) First 1^(st) 149 Extruder 2^(nd) 170 3^(rd) 171 4^(th) 224 5^(th)200 6^(th) 191 Guide 250 Second 1^(st) 170 Extruder 2^(nd) 150 3^(rd)145 4^(th) 140 5^(th) 140 6^(th) 135 Gear pump 134 Homogenizing device130

EXAMPLE 7

The temperature conditions in the device were set as described in Table7 and the pellet-type foams were prepared by using the same procedureand materials as in Example 1.

TABLE 7 Temperature condition Temperature Temperature Device Zone No. (°C.) First 1^(st) 150 Extruder 2^(nd) 170 3^(rd) 170 4^(th) 220 5^(th)200 6^(th) 190 Guide 250 Second 1^(st) 167 Extruder 2^(nd) 152 3^(rd)142 4^(th) 141 5^(th) 137 6^(th) 132 Gear pump 134 Homogenizing device130

COMPARATIVE EXAMPLE 1

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except that the homogenizing device wasnot operated. The homogenizing device was maintained at a temperature of130° C. but was not activated. Therefore, the melt from the gear pumppassed through the temperature zone of 130° C. without homogenization tobe introduced to the dies.

COMPARATIVE EXAMPLE 2

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the first temperature zone ofthe first extruder, which was set to 146° C.

COMPARATIVE EXAMPLE 3

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the second temperature zone ofthe first extruder, which was set to 173° C.

COMPARATIVE EXAMPLE 4

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for third temperature zone of thefirst extruder, which was set to 173° C.

COMPARATIVE EXAMPLE 5

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fourth temperature zone ofthe first extruder, which was set to 226° C.

COMPARATIVE EXAMPLE 6

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fifth temperature zone ofthe first extruder, which was set to 205° C.

COMPARATIVE EXAMPLE 7

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the sixth temperature zone ofthe first extruder, which was set to 187° C.

COMPARATIVE EXAMPLE 8

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the guide, which was set to256° C.

COMPARATIVE EXAMPLE 9

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the first temperature zone ofthe second extruder, which was set to 174° C.

COMPARATIVE EXAMPLE 10

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the second temperature zone ofthe second extruder, which was set to 153° C.

COMPARATIVE EXAMPLE 11

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the third temperature zone ofthe second extruder, which was set to 148° C.

COMPARATIVE EXAMPLE 12

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fourth temperature zone ofthe second extruder, which was set to 142° C.

COMPARATIVE EXAMPLE 13

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the fifth temperature zone ofthe second extruder, which was set to 143° C.

COMPARATIVE EXAMPLE 14

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the sixth temperature zone ofthe second extruder, which was set to 138° C.

COMPARATIVE EXAMPLE 15

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the gear pump, which was setto 141° C.

COMPARATIVE EXAMPLE 16

Pellet-type foams were prepared by using the same procedure andmaterials as in Example 1 and setting the temperature conditions in thedevice as described in Table 1 except for the homogenizing device, whichwas set to 131° C.

EXPERIMENTAL EXAMPLE 1

The pellet-type foams obtained from Example 1 and Example 2 weremeasured for DSC (differential scanning calorimeters; 10° C./min. to200° C., 50 cc/min. to N₂ purge) transition temperature according to thetest method KSM3050-2001. The results are shown in FIG. 11 and FIG. 12.As shown in the DSC curves of FIG. 11 and FIG. 12, the foams of Example1 and Example 2 had melting points of 137.62° C. and 128.38° C.,respectively, which are lower than the melting point of 138° C. of therandom copolymer (RP2400 (polypropylene-polyethylene (3%) randomcopolymer) used as the row material. RP2400 random copolymer as controlwas measured for the DSC transition temperature. The results are shownin FIG. 13.

EXPERIMENTAL EXAMPLE 2

The pellet-type foams obtained from Example 1 and RP2400(polypropylene-polyethylene (3%)) random copolymer as control weresubject to the elementary analysis. The analysis was performed by usingthe CE EA-1110 elementary analyzer. The results are shown in Table 8given below.

TABLE 8 Results of the elementary analysis analysis item Sample C (%) H(%) N (%) RP2400 random copolymer 85.1 15.0 N.D.* Pellet-type foams ofExample 1 84.2 14.8 0.5 N.D. means “non-detectable”. The detecting limitof N is 0.1%.

EXPERIMENTAL EXAMPLE 3

The pellet-type foams obtained from Example 1 and RP2400(polypropylene-polyethylene (3%)) random copolymer as control weresubjected to the FT-IR analysis. The analysis was performed by using theBio-Rad Digilab FTS-165 FT-IR Spectrophotometer. The results are shownin FIG. 14 and FIG. 15, respectively. According to the results, it isnoted that the pellet-type foams obtained from Example 1 and RP2400random copolymer have a main component of polypropylene.

EXPERIMENTAL EXAMPLE 4

The open cell contents of the pellet-type foams prepared in the Examplesand Comparative Examples were measured according to the procedure Cdescribed in the ASTM D 2856-70. The results are shown in Table 9 givenbelow.

TABLE 9 Open cell No. content Example 1  6% 2 13% 3 12% 4 16% 5 14% 6 7% 7 12% Comparative example 1 68% 2 49% 3 52% 4 38% 5 44% 6 39% 7 52%8 40% 9 37% 10  36% 11  31% 12  53% 13  29% 14  28% 15  26%

EXPERIMENTAL EXAMPLE 5

Other physical properties of the foams prepared in Example 1 weremeasured according to a conventional method. The results are shown inTable 10 given below.

TABLE 10 Item Foams of Example 1 Magnification 25 Appearance color Lightivory Cell Size 300 μm Compressive strength 0.81 kgf/cm²

EXPERIMENTAL EXAMPLE 6

The pellet-type foams obtained from Example 1 was molded by using amolder, 500GF 4, produced by Daekong Machinery Co., Ltd., with a moldingpressure set to 2.5 kgf/cm² (temperature of about 138° C.), to produce awell-molded article. Through production of such a well-molded article,it is proved that the pellet-type foams according to the presentinvention are melted at a temperature of 138° C. to adhere to eachother, leading to sold bonding between foams.

EXPERIMENTAL EXAMPLE 7

The pellet-type foams obtained from Example 2 was molded by using amolder, 500GF 4, produced by Daekong Machinery Co., Ltd., with a moldingpressure set to 2.4 kgf/cm² (temperature of about 132° C.), to produce awell-molded article. Through production of such well-molded article, itis proved that the pellet-type foams according to the present inventionare are melted at a temperature of 132° C. to adhere to each other,leading to solid bonding between foams.

From the above description, it will be appreciated that the presentinvention can be embodied as other specific forms by those skilled inthe art without departing from the spirit and indispensable features ofthe present invention. With regard this, it should be understood thatthe examples and experimental examples described above have been made byway of example and not as a limitation. It should be construed that allmodification and change derived from the meaning and scope of the claimsdescribed below and equivalent rather than the above description andequivalents thereof are intended to be included within the scope of thisinvention.

The pellet-type foams of non-crosslinked polypropylene according to thepresent invention has a closed cell content of 80% or more and a meltingpoint of 125 to 140° C. so that the pellet-type foams of the inventionis much available in a standpoint of formation and regeneration.

1. A device for producing a pellet-type non-crosslinked polypropylenefoam having a melting point of 125 to 140° C., comprising a firstextruder, a second extruder connected to the first extruder, a pumpingpart connected to the second extruder, a homogenizing part connected tothe pumping part and a dies part connected to the homogenizing part,wherein the first extruder comprises a cylindrical cylinder in which ascrew shaft is mounted rotatably, a driving controller installed at anend of the cylinder for rotating the screw shaft and a plurality ofcooling water passages and heaters that are alternately mounted on anouter circumference of the cylinder, wherein each set of alternatingcooling water passages and heaters form a specific temperature zone, thecylinder has entrances formed at an end portion thereof corresponding tothe driving controller for supplying a raw polypropylene and anucleating agent thereto, an exit formed at another end portion thereof,entrances formed at a midportion thereof for supplying an additive and afoaming agent, respectively, whereby the raw material supplied to thecylinder via the entrances is flow compulsorily toward the exit by thescrew shaft rotated according to the driving controller; the secondextruder comprises a cylinder into which the polypropylene meltdischarged from the cylinder of the first extruder is supplied throughthe guide, and a plurality of cooling heating units mounted to an outercircumference surface of the cylinder for adjusting a temperature of thepolypropylene melt in the cylinder; the pumping part comprises a casinghaving an inner space into which the polypropylene melt discharged fromthe cylinder of the second extruder is supplied, a pair of gearsinstalled rotatably and meshed with each other in the inner space of thecasing and a driving unit for rotating the gears, whereby thepolypropylene melt supplied therein is flow compulsorily; thehomogenizing part comprises a first cylindrical housing into which thepolypropylene melt discharged from the cylinder of the pumping part issupplied, the first housing being installed rotatably, a driving elementfor rotating the first housing, a screw connected to an exit end of thefirst housing, a second housing located on an outer circumference of thescrew, a frame mounted to an outer circumference of the second housingfor forming an airtight space between the second housing and the frame,and a homogenizer installed at a rear end of the second housing; whereina spiral space is formed between the screw and the second housing alongthe entire length of the screw so that the polypropylene melt dischargedfrom the first housing is flow in the space formed between the screw andthe second housing and discharged to an outside, a heat transfer fluidoil flows in a space formed between the second housing and the frame toadjust a temperature of the polypropylene melt which flows in the secondhousing, and the homogenizer crushes the polypropylene melt; the diespart into which the polypropylene melt discharged from the homogenizingpart is supplied includes a discharging part, a cutting part, and adriving member so that expanded foams are cut at a certain size.
 2. Thedevice for producing a pellet-type foam of claim 1, wherein each coolingwater passages mounted to the cylinders of the first and secondextruders is a casing in which a cooling water supplied from an outsideflows, the cooling water supplied to the casing is contacted with asurface of the cylinder and flow along the casing so that a temperatureof the polypropylene melt that flows in the cylinder becomes lower, eachheater mounted between two casings is a heater in which a heating coilis installed.
 3. The device for producing a pellet-type foam of claim 1,wherein the cylinder of the first extruder is divided into sixtemperature zones according to the temperature condition of thepolypropylene melt supplied to the cylinder, a temperature of each zoneis adjusted by the cooling water passages and heaters mounted to anouter circumference of the cylinder, a temperature of the polypropylenemelt is 147 to 153° at the first temperature zone, 167 to 172° C. at thesecond temperature zone, 168 to 172° C. at the third temperature zone,218 to 225° C. at the fourth temperature zone, 197 to 203° C. at thefifth temperature zone and 188 to 193° C. at the sixth temperature zone.4. The device for producing a pellet-type foam of or claim 3, whereinthe entrances for supplying a raw polypropylene and a nucleating agentare formed on the first temperature zone of the cylinder of the firstextruder, the entrance for supplying antistatic agent is formed on thethird temperature zone, and the entrance for supplying a foaming agentis formed on the fourth temperature zone.
 5. The device for producing apellet-type foam of claim 4, wherein the foaming agent to be supplied tothe cylinder of the first extruder is CO₂ discharged from a device forsupplying CO₂, the device for supplying CO₂ comprises a tank for storingCO₂, an unit for vaporizing and freezing CO₂ connected to the tank, astabilizing unit for transforming CO₂ supplied from the unit forvaporizing and freezing into a vapor; and a storage unit for storing CO₂supplied from the stabilizing unit to supply CO₂ into the first cylinderof the first extruder.
 6. The device for producing a pellet-type foam ofclaim 1, wherein the cylinder of the second extruder is divided into sixtemperature zones according to the temperature condition of thepolypropylene melt supplied to the cylinder, a temperature of each zoneis adjusted by the cooling water passages and heaters mounted to anouter circumference of the cylinder, a temperature of the polypropylenemelt is 167 to 173° C. at the first temperature zone, 147 to 152° C. atthe second temperature zone, 142 to 147° C. at the third temperaturezone, 137 to 141° C. at the fourth temperature zone, 137 to 142° C. atthe fifth temperature zone and 132 to 137° C. at the sixth temperaturezone.
 7. The device for producing a pellet-type foam of claim 1, whereinthe two gears mounted in the inner space of the casing of the pumpingpart are rotated in an opposite direction from each other toward acenter of the inner space to make the polypropylene melt movecompulsorily to a next process position.
 8. The device for producing apellet-type foam of claim 1, wherein the first housing of thehomogenizing part is supported rotatably to support plates through aplurality bearing blocks, a drive sprocket of the driving element isgeared with a driven sprocket fixed to an outer circumference surface ofthe first housing so that the first housing is rotated in response of anoperation of the driving element.
 9. The device for producing apellet-type foam of claim 1, wherein the homogenizer of the homogenizingpart is consisted of a rotating plate mounted rotatably and a fixingplate contacted with the rotating plate, the rotating plate has aplurality of openings arranged radially and the fixing plate has aplurality of circular holes, whereby after the supplied polypropylenemelt is arrived at the rotating plate and passed through each openingformed on the rotating plate, the polypropylene melt is cut by an edgeof each opening of the rotating plate, and then the crushedpolypropylene melt is ground between the rotating plate and the fixingplate by the rotating plate.
 10. The device for producing a pellet-typefoam of claim 1, wherein the discharging part of the dies part isconsisted of a hollow guide bar and a cylinder located on an outside ofthe guide bar, the guide bar has a plurality of cavities formed on theouter circumference thereof in a longitudinal direction of the guide barso that the polypropylene melts discharged from the homogenizing partflows in each cavity, a plurality of through holes are formed atportions of the cylinder which are corresponded to the cavities,respectively; the cutting part comprises a supporting plate located at arear side of the discharging part and cutting members fixed to thesupporting plate and located movably on an outside of the cylinder, eachcutting member has a plurality of through holes formed at positionswhere are corresponded to the plurality of through holes of thecylinder, respectively; and the driving member comprises an eccentriccam being able to rotate by a motor, a crank connected to the eccentriccam and rotated in response to a rotation of the cam and a powerconverting and transmitting unit connected to the crank for converting arotating movement of the crank into a linear movement and transmittingthe linear movement to the supporting plate to which the cutting memberis fixed; whereby each cutting member is reciprocated along the outercircumference of the cylinder according to an operation of the drivingmember so that the polypropylene foams expanded through each throughhole of the cylinder is cut by edges of the through holes of the cuttingmember.
 11. The device for producing a pellet-type foam of claim 10,wherein a diameter of each through hole formed on the cylinder is lessthan that of each through hole of the cutting member, each through holeof the cylinder is located on a central portion of the correspondingthrough hole of the cutting member at an initial position.
 12. Thedevice for a producing pellet-type foam of claim 10, wherein thecylinder has a plurality of grooves are formed an outer circumferencethereof in a longitudinal direction, reciprocating rods whose ends arefixed to the support plate are located movably in the grooves,respectively, and each cutting member is fixed to each reciprocating rodby fixing elements so that each cutting member is reciprocated on anouter circumference of the cylinder by the each reciprocating rod whichis reciprocated along each groove of the cylinder.
 13. The device forproducing a pellet-type foam of claim 10, wherein the dies part furthercomprises a decompression unit for preventing a rapid change of atemperature and pressure to the expanded and extruded foams, thedecompression unit is a casing which is able to isolate the dischargingpart and cutting part from an outside atmospheric, the casing has anentrance port through which an air is flow and an exit port throughwhich an air is exhausted at both sides.
 14. The device for producing apellet-type foam of claim 10, wherein the dies part further comprises acooling device for cooling the cylinder, the cooling device comprises; asupplying pipe into which a heat transfer fluid oil is supplied from anoutside, the supplying pipe being located at a front of the cylinder; aplurality of flowing pipes having entrance ends connected to thesupplying pipe and mounted in the cylinder along the entire length ofthe cylinder; and a discharging pipe being located at a front of thecylinder and connected to exit ends of the flowing pipes for receivingthe heat transfer fluid which exchanged a heat with the cylinder.